This invention relates to a method of and apparatus for compressing form data employed, for instance, for a COM (computer output microfilming) device.
The COM device, to which the technical concept of the invention is effectively applicable, is used to record variable data, such as incoming and outgoing data entered in a business book, on a recording medium in combination with fixed data, such as those of forms with lines as included in a business book (cf. "Micrographic Systems" (second edition) by Daniel M. Costigan published from NATIONAL MICROGRAPHICS ASSOCIATION, and U.S. Pat. No. 4,293,202.
In one example of the method, variable data recorded on a recording medium, such as a recording magnetic tape, are processed by computer for display on the picture plate of a cathode ray tube. The picture thus displayed is superposed on the picture obtained by optically reading the fixed data recorded on a form slide. The combined display is recorded as a frame on a microfilm. In another example of the method, a video signal of the variable data, obtained by computer-processing, is recorded on a recording medium in combination with a video signal of the fixed data which are obtained by scanning a form slide with a laser beam.
However, these conventional methods are disadvantageous in that, as the form slide is optically read, in recording the picture, the latter is liable to be shifted or distorted.
The difficulty may be eliminated by employing a method in which, instead of directly reading the fixed data of the form slide during the recording operation in the COM device, the fixed data of the form slide are stored as electrical signals in a memory, and are then read out of the memory during the recording operation.
In general, recording pictures for a COM device have at least about 2000.times.2500 resolution points per frame. Accordingly, it is difficult to provide a memory which has a sufficient number of memory locations equal to the number of resolution points. If a plurality of memories are provided to cover all the resolution points, then the device becomes large, complex and expensive.
On the other hand, the form data comprises: framing data such as those of ruled lines; and fixed data such as those of customer names and code numbers which are to be recorded at predetermined positions. Clearly, less form data must be recorded to produce a form picture since much of the form is blank and a significant part of the form data is redundant (blank data). Therefore, if a method is employed in which the form data, after being compressed, are stored in memory, and the memory is operated together with the COM device to expand the data, so that the composite picture of the above-described variable data and the form data is recorded, then it is possible to miniaturize the COM device and to obtain a variety of advantages, such as, a reduction of the manufacturing cost.
Examples of conventional data compressing methods are a run length method, a plane coding method, and a change point address coding method. All of these methods are generally employed in the field of communication; however, these methods are disadvantageous in that they are intricate, and have a low processing speed. In the case of a COM device, the data should be processed at least at a rate of 30 ns per picture element; however, in the case of the run length method, there is insufficient time for compressing and expanding the data in such a short period.